The present invention relates to an improvement in a process for the production of chlorine and caustic soda using a membrane electrolysis cell coupled to an alkaline fuel cell.
About ten percent of all electric power produced in the United States is consumed by the electrochemical industries. The production of chlorine and caustic soda accounts for about 20% by this consumption. An energy consumption of about 287,000 bbl of oil per day are used in the daily production of approximately 37,000 tons of chlorine. Improving the efficiency of this process will have a significant impact on energy consumption as well as the financial performance of the relevant industry.
Three types of electrolysis cells currently dominate the electrolytic chlorine-caustic industry: (a) mercury, (b) diaphragm and (c) membrane cells. Mercury cells are finding increasingly limited application because of pollution problems and will most certainly be phased out in the near future. Diaphragm cell technology presently accounts for 75% of the U.S. electrolytic chlorine-caustic production. Membrane cell technology is an emerging technology which is being studied extensively in Japan.
In the diaphragm process the cell is divided into anode and cathode compartments by an asbestos diaphragm. Feed brine, i.e. NaCl solution, enters the anode compartment where chlorine gas is liberated. The brine convects through the diaphragm into the cathode compartment in which H.sub.2 is produced. The anode and cathode reactions are: EQU 2Cl.sup.- .fwdarw.Cl.sub.2 +2e.sup.- (anode) EQU 2H.sub.2 O+2e.sup.- .fwdarw.2OH.sup.- +H.sub.2 (cathode)
Sodium ions flow through the asbestos diaphragm by convection and under the influence of an imposed electric field. The catholyte typically exits the cell with 10 to 15 wt. percent NaOH and nearly saturated brine. The caustic is purified in a series of evaporation procedures to approximately 50 wt. percent NaOH. The H.sub.2 gas produced at the electrolysis cell cathode may be used in several ways as an energy source or as a commercial product.
In the diaphragm process, two steps using approximately equal energy, require the greatest energy inputs--(1) brine electrolysis and (2) caustic purification and concentration.
In the membrane process the electrolysis cell is divided into anode and cathode compartments by a cation-transporting membrane which prevents transport of anions across the membrane, but not cations. The electrode reactions are the same as for the diaphragm cell. The catholyte effluent generally consists of 25 to 30 wt. percent NaOH with approximately 50 ppm NaCl. Again, H.sub.2 is produced as a by-product. The most significant differences between the membrane cell and the diaphragm cell are that the NaOH produced from the membrane electrolysis cell contains only trace amounts of NaCl and that less energy is required with the membrane cell for caustic concentration.
Using fuel cells to take advantage of the hydrogen produced during brine electrolysis has been proposed. "Proceedings of the Workshop on Energy Conservation in Industrial Electrochemical Processes," Aug. 10-12, 1976, ANL/OEPM-77-1, Argonne National Laboratory, Contract W-31-109-Eng-38, Ed. N. P. Yao et al.; Beck, T., "Final Report on Improvements in Energy Efficiency of Industrial Electrochemical Processes, "ANL/OEPM-72-2, Electrochemical Technology Corp., for Argonne National Laboratory, Contract W-31-109-Eng-38, Jan. 1977; Greek, B. F. and Fallwell, W. F., Chemical and Engineering News, Mar. 1, 1982, p. 10. Fuel cells are direct conversion devices that convert chemical energy to electrical energy directly by electrochemical reactions. Electricity is generated by the electrochemical oxidation of hydrogen at the cell anode and electrochemical reduction of oxygen at the cathode. The specific electrode reactions depend on the fuel cell electrolyte used. The electrolyte, however, does not participate in the net cell reaction and is thus unaffected. The electrolyte serves the function of transferring electrons between the anode and cathode by ionic transport.
Alkaline fuel cells use aqueous NaOH or KOH for electrolytes. Their half call reactions may be written as: EQU anode: 1/2H.sub.2 +OH.sup.- .fwdarw.H.sub.2 O+e.sup.-
and EQU cathode: 1/2O.sub.2 +H.sub.2 O+2e.sup.- .fwdarw.2OH.sup.-.
The net cell reaction is the oxidation of hydrogen to water: EQU H.sub.2 +1/2O.sub.2 .fwdarw.H.sub.2 O.
Alkaline fuel cells operate at low temperatures in the range of about 60.degree. to 100.degree. C.
Recently, a coupled diaphgragm/alkaline fuel cell system capable of reducing the process energy consumption has been disclosed. (U.S. Pat. No. 4,246,078 issued Jan. 20, 1981). In this approach, an alkaline fuel cell, operating on sodium hydroxide, is coupled to a diaphragm electrolysis cell to both provide process power and to electrochemically separate the caustic from the contaminant brine. While this process could potentially eliminate most of the boiler energy used to concentrate caustic and reduce the electrical power requirement, several drawbacks exist. Some of the product caustic produced by the electrolysis cell is returned to the brine processing step. This portion of the caustic product is thereby lost to the system because the brine must be acidified before entering the electrolysis cell. Moreover, since the electrolysis cell anolyte must be acidic there is a large increase in the requirement for HCl to neutralize the caustic that is returned to the brine. The HCl for acidification requires the consumption of the hydrogen and chlorine products. Hence this results in a significant decrease of both hydrogen and chlorine product yields. An additional difficulty is the level of impurities in the diaphragm cell process. These impurities severely limit the alkaline fuel cell membrane lifetime.
U.S. Pat. No. 4,246,078 issued Jan. 20, 1981 discloses a hybrid cell where a catholyte, an alkali hydroxide solution, preferably a solution of NaOH and NaCl, from the cathode of an electrolysis cell, is introduced into the anolyte chamber of an alkaline fuel cell. An aqueous medium, water or a dilute alkali metal hydroxide solution, is fed into the fuel cells catholyte chamber. The alkaline fuel cell has a cation selective permeable membrane separating the two compartments. Thereafter, by introducing hydrogen to the anolyte compartment, and a flow of air to the catholyte compartment, electrical energy is generated with the following reactions occurring:
(a) anode: H.sub.2 +OH.sup.- .fwdarw.H.sub.2 O+e.sup.-, PA0 (b) cathode: H.sub.2 O+1/2O.sub.2 +2e.sup.- .fwdarw.2OH.sup.- ; while cations, e.g., Na.sup.+, from the anolyte chamber pass through the membrane and into the catholyte chamber. By this method a concentrated solution up to 50% NaOH is stated to be extracted from the cathode. PA0 (a) feeding a brine to the anolyte compartment solution and a dilute NaOH solution to the catholyte compartment of an electrolysis cell comprising a cation-transporting membrane, thereby generating chlorine at the anode, hydrogen at the cathode, and concentrating NaOH in the catholyte solution; PA0 (b) coupling the membrane electrolysis cell to a cation-transporting membrane alkaline fuel cell by providing at least some of the hydrogen generated in the electrolysis cell to the anode of the fuel cell, and splitting the catholyte flow from the catholyte compartment of the electrolysis cell so that it feeds into both the catholyte chamber and the anolyte chamber of the fuel cell in an unequal amount with a greater proportion of the catholyte going into the anion chamber; PA0 (c) concentrating the NaOH in the fuel cell by providing an oxygen source to the catholyte chamber of the fuel cell, and using the oxygen and the hydrogen to generate a current in the fuel cell, thereby causing Na.sup.+ to be transported across the membrane and concentrating NaOH in the catholyte chamber; and PA0 (d) collecting the chlorine from the electrolysis cell and the concentrated NaOH from the fuel cell.
Although a concentrated, caustic solution is obtained at the cathode of the fuel cell, some caustic still remains in the anode compartment. While it is theoretically possible to remove substantially all the alkali metal hydroxide from the anolyte, such a process involves operating the fuel cell under short-circuited conditions producing no usable current. Therefore, the solution from the anolyte compartment preferably contains NaCl and some NaOH. This brine solution contaminated with caustic thus becomes a waste product which must be discarded, unless acidified with HCl resulting in loss of product H.sub.2 and Cl.sub.2.
Thus, further improvements in the production of chlorine and caustic soda are still being sought.